processes for general shapes

Waterless Molding - aims to eliminate the sometimes detrimental effects of moisture in the molding sand. Clays are treated to react with oils rather than water to-de them bond to the sand particles. The hot strength of the waterless-bonded sand is somewhat lower than that of conventional green sands. This reduces the force needed to displace the sand as the casting shrinks during solidification, which in mm reduces the potential for hot tearing. On the other hand, sands with low hot strength have a greater tendency to be damaged by hot metal flowing into the mold.'

For large castings, molds may be baked or partially dried to increase their strength. The surfaces of skin-dried molds are treated with organic binders, and then dried by means of torches or heaters. To make dry sand molds, simple organic bonding agents such as molasses are dissolved in the bonding water when making up the green sand mixture. The entire mold is then baked to develop the desired hot strength. Besides hardening the mold, removing water also reduces the chance for blowholes and other moisture-related casting defects. Baking and skin drying are expensive operations and the dry sand methods are rapidly being replaced by a variety of no-bake processes, described below.

There are three general types of low-temperature-curing, chemical binders: Cement has traditionally been used as a bonding agent in the extremely large molds used to cast marine propellers and similar products. Cement bonded molds are extremely strong and durable, but they must be designed carefully since their inability to yield under solidification shrinkage stresses may cause hot tearing in the casting.

Organic binders utilize resins that cure by reaction with acidic catalysts. Furan-, phenolic-, and urethane-base systems are the most popular of the large variety of currently available bonding agents. Of the inorganic binders, the well-known liquid sodium silicate-CO2 process is most widely used for copper alloy castings.